Two new projects on primate play, utilizing the squirrel monkey as a model, were begun this year. Data collection on both was completed in FY86. A. Social Environment Effects on Play. This study investigates possible differences in the frequency and/or style of play when young squirrel monkeys are experimentally restricted to particular social companions from whom they differ with respect to age, size, or sex. Our basic strategy is to allow subject youngsters the opportunity to play only with particular partners with known play profiles. This new approach promises to yield differences in play experience without producing the devastating effects of depriving animals of social contact with peers, a flaw in previous studies performed elsewhere that has made their results ambiguous at best. The primary objective of this study is to learn the effect of different play experiences on later social outcomes. The importance of reciprocity (role reversal) in sustaining mutually beneficial play relationships may be reflected in the later development of normal and disordered cognition and behavior. B. Vocalizations Used in Play. Squirrel monkeys are one of only a few species (including man) having a prominent and specific play vocalization, providing a good window for investigation of motivational changes occurring during play. Two main call types, one with four variants, were identified by sound spectographic analysis. Vocalization rate varied with the type of ongoing behavior and with play bout duration, an association considered to be indicative of motivation to play. Structural differences also varied with bout duration, in that longer play bouts were associated with longer and more complex calls. In addition to providing direct information about motivation, vocalizations may also carry a metacommunicative message to nearby adults that the play activity is relatively harmless.